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Looking at the MAME driver for the Apple //e, I noticed an interesting comment:

NOTE: On real IIe and IIe enhanced h/w, pressing SHIFT and paddle button 2 will short out the power supply and cause a safety shutdown. (We don't emulate this "feature", and it was relatively rare in real life as Apple joysticks only had buttons 0 and 1 normally).

I've never heard of this before, though I don't recall ever utilizing paddle button 2 either.

Is this an actual problem that always existed? Would reproducing it by hooking up the right pins on the Game I/O port be unsafe or potentially damaging to the //e or is it completely harmless because the circuitry is designed to handle such an event? Why does this "feature" exist?

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    I'm pretty sure only the Platinum //e had this problem out of the box as it was the only model that shipped with the shift key mod installed. I note this and link to some references (in a comment) in this answer.
    – Nick Westgate
    May 30, 2019 at 22:05

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The keyboard on the Apple II+ was designed to type uppercase letters only, which rather limited the machine's usefulness for word processing. Because almost nothing used paddle button 2 and very few controllers even had a third button, a common user-installed modification was to run a wire from the shift-key signal to the paddle-button-2 input. Word processing programs could then be set to use paddle button 2 to check whether the shift key was down and use that to decide whether to type uppercase or lowercase. This was far from perfect, since if e.g. a user typed shift+A and then released both keys while the system was doing something, the system would then poll the keyboard, find out an "A" had been pushed, observe that the shift key was not (still) pressed, and type a lowercase "a". It was nonetheless superior to being unable to type lowercase letters at all, or requiring that users press ESC before any uppercase letter (the other approach word processors used).

Although the Apple //e allows lowercase letters to be typed directly, the designers wanted to be compatible with code that expected the shift-key mod to be installed, so they simply included the connection on the motherboard directly.

The problem is that paddle buttons were intended to be used by wiring a resistor that would weakly pull the input to +5V when the button isn't pushed, and then having the switch pull the input to ground when pressed. The shift key was wired the opposite way--passively pulled down, with a switch to +5V. If Apple had foreseen the potential issue this could cause, it could have been averted by simply adding a resistor in series with the switch. Alternatively, Apple could have included a jumper that would connect that input to either the appropriate pin on the internal controller socket or to the shift key. My guess is they simply failed to recognize the possibility of a problem since paddle 2 inputs would be used so rarely. It was certainly never intended as a "feature".

As for whether this would cause damage, it would certainly cause excessive stress on the switch contacts used for the shift key and/or paddle 2 input, and would likely stress the power supply components as well. Doing this once or twice might not cause the components to die instantly, but one probably wouldn't have to do it too many times to wear out the components.

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    Any idea why the MAME team would call this a "safety shutdown"? Is that a term of art I'm unfamiliar with? The whole thing sounds decidedly unsafe from the HW's point of view.
    – Tommy
    May 30, 2019 at 17:13
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    @Tommy: Power supplies can be designed to behave in various ways when overloaded. Some will feed the load as much current as they are capable of supplying, some will fail destructively (typically by popping a fuse), and some will shut down non-destructively. Short-circuiting the supply is dangerous, but having the power supply shut down in response to a short-circuit condition is safer than having it simply try to feed as much current as it can.
    – supercat
    May 30, 2019 at 17:36
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    If only for the addition of a single little resistor, this could have been Apple's version of spacebar heating...
    – J...
    May 31, 2019 at 18:15
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    @Tommy At least in the PC world power supplies monitor their output voltages and assert a voltage-good signal. Once it's had time to turn on the lack of that signal causes a safety shutdown--your machine turning off suddenly is better than your machine going wonky and perhaps corrupting the hard drive.
    – Loren Pechtel
    May 31, 2019 at 22:09
  • As per my comment on the question, this only affects the Platinum //e out of the box.
    – Nick Westgate
    Jun 5, 2019 at 2:10

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